Semiconductor laser diodes have become important components in the technology of optical communication, particularly because such laser diodes can be used for amplifying optical signals immediately by optical means. This allows for the design of all-optical fiber communication systems, avoiding complicated conversions of the signals to be transmitted. The latter improves speed as well as reliability within such communication systems.
In one kind of optical fiber communication system, the lasers are used for pumping erbium-doped fiber amplifiers, so called EDFAs, which have been described in various patents and publications known to the person skilled in the art. An example of some technical significance are 980 nm lasers with a power output of 100 mW or more, which wavelength matches the 980 nm erbium absorption line and thus achieves a low-noise amplification.
A conventional design of a laser device 1 is illustrated in FIG. 1. Here a semiconductor laser 11 includes a waveguide 20, rear facet 16, and front facet 18. The semiconductor laser 11 is combined with an optical fiber 14 to effectively guide the light through a partially reflecting, wavelength selective reflector 26 to an optical amplifier (not illustrated). The optical fiber 14 includes a fiber lens 22 and a lens tip 24 thereof. Light from waveguide 20 is incident upon the fiber lens 22 at the lens tip 24. The optical fiber 14 produces a few percent of feedback and locks the laser device 1 to the prescribed wavelength of the wavelength selective reflector 26. Descriptions of such a design can be found, for example, in U.S. Pat. No. 7,099,361 and in US Patent Application Publication No. 2008/0123703. This design provides for a laser without the need for an active temperature stabilizing element, as the stabilization by a wavelength selective reflector 26 constituted as a fiber Bragg grating (FBG) yields low temperature sensitivity of the wavelength shift, typically by about 7 pm/° K, which cannot be achieved by a grating inside the semiconductor laser 11 (DBR or DFB structure).
The effects of constructive and destructive interference between residual reflections of laser front facet 18 and fiber lens 22 were previously investigated for the laser device 1 illustrated in FIG. 1 but without a wavelength selective reflector 26 in “Impact of near-end residual reflectivity on the spectral performance of high-power pump lasers”, IEEE Journal of Quantum Electronics, April 2004, Volume: 40, Issue 4, pp. 354-363. The study revealed that even for standard lenses and laser facets both having AR coatings much lower than 1%, the effective laser front reflectivity from combined reflection varies with different operating conditions, i.e. changes with temperature and laser current. As a result, discontinuities in the laser spectrum can be observed. For state-of-the-art lasers at the time of the study which had a shorter cavity length, i.e. less roundtrip gain and less coherence, these effects were negligible when wavelength stabilization by a FBG was applied.
But it has been found by the inventors that conventional laser devices 1 such as that illustrated in FIG. 1 having a semiconductor laser 11 with a longer cavity (e.g., greater than 3 mm) and that produce a high amount of gain are more susceptible to the effects from residual reflections and/or feedback of any reflectors in the optical path, as well as effects of additional Fabry-Perot (FP) cavities formed between multiple reflectors. As illustrated in FIG. 6A, such effects can produce a high amount of unwanted ripple in the optical power versus current characteristic. Even a small amount of back reflection from an AR coated lens tip 24 or AR coated front facet 18 into the semiconductor laser 11, which produces a high amount of gain, can have large impact. The laser output can also become very sensitive to subtle changes to optical coupling.
It is known that curved waveguides with tilt angles, e.g. more than 2° with respect to a face of the front facet, can suppress back reflections into the waveguide. Such an arrangement reduces optical feedback (i.e., back reflections) from the front facet 18 as the radiation reflected from the front facet 18 does not couple into the active waveguide 20 itself. Accordingly, other conventional designs incorporate a curved waveguide 20 which forms effectively a semiconductor gain element 12, rather than a semiconductor laser 11, in the absence of a front facet feedback. In the conventional laser device 5 illustrated in FIG. 2, a laser cavity is only established by providing an additional feedback element into the optical path, which is the wavelength selective reflector 26 in this case. Here the semiconductor gain element 12 is combined with an optical fiber 14 having a conventional fiber lens 22. The conventional fiber lens 22 includes a lens tip 24 that is orthogonal to the longitudinal axis of the optical fiber 14. The fiber lens is arranged such that optical radiation emitted from the semiconductor gain element 22 (i.e., the propagation direction of optical radiation from the semiconductor gain element 22) is orthogonal to the lens tip 24.
The laser device 5 of FIG. 2 provides some improvement over the laser device 1 of FIG. 1 (see FIG. 6B), and has previously been applied with some success for semiconductor gain elements with low gain (e.g., Wavelength stable uncooled fiber grating semiconductor laser for use in an all optical WDM access network”, IEEE Electronics Letters, 18 Jan. 1996, Volume: 32 Issue: 2, pp. 119-120). However, the ripples in the power versus current characteristics cannot be fully suppressed. That is, while the FP cavity formed between the rear facet 16 and front facet 18 in FIG. 2 is spoiled by using the curved waveguide 20, and while the additional cavity that appears between fiber lens 22 and front facet 18 is significantly suppressed by tilting both the emitted optical radiation and the fiber lens, the FP cavity formed between the rear facet 16 and the fiber lens 22 remains, contributing to unwanted ripples in the power versus current characteristic even if an AR coating is applied on the fiber lens tip 24.